The present invention pertains to an apparatus for forming a single face web of corrugated paperboard and, more particularly, to a corrugating roll assembly for a single facer.
In the manufacture of corrugated paperboard, a single facer apparatus is used to corrugate the medium web, apply glue to the flute tips on one face thereof, and to bring a liner web into contact with the glued flute tips of the medium web with the application of pressure and downstream heating to provide an initial bond. A conventional single facer typically includes a pair of fluted corrugating rolls and a pressure roll, which are aligned so the axes of all three rolls are generally coplanar. The medium web is fed between the inter-engaging corrugating rolls and the adhesive is applied to the flute tips by a glue roll while the medium is still on the corrugating roll. The liner web is immediately thereafter brought into pressurized contact with the adhesive-coated flute tips in the nip between the pressure roll and the corrugating roll.
As corrugating nip roll pressures and corrugating speeds have increased, changes have been made in the construction of single facers to maintain the quality of the corrugated medium and to attempt to deal with the problems of high noise and vibration. For example, the load between corrugating rolls at the corrugating nip has required that one of the fluted corrugating rolls be made with a crowned surface to accommodate roll deflection under high nip loads. Deflection as a result of high loading is also believed to be one source of noise and vibration. In a conventional single facer construction, where the two corrugating rolls and the lower pressure roll are in general alignment (their axes lying generally coplanar), corrugating roll loads are transmitted to the pressure roll adding further to the problems associated with high loads and high speeds. This has resulted, in some cases, in manufacturing the pressure roll with a negative crown to match deflections in the corrugating roll which together form the nip for joining the two single face web components.
One of the most serious problems in the operation of high speed single facers is the stress applied to the medium web and liner web as the liner is introduced into contact with the medium web under pressure. Because of this pressure induced stress on the corrugated web, the possible thickness of the corrugated paperboard currently is limited such that the individual web components are thick enough to withstand the stress and resist tearing. Tests done with lighter weight web components have shown that the current techniques literally shred the medium web along the lines of the row teeth of the corrugating rolls due to the high pressure between the corrugating roll and the pressure roll containing the liner.
It has long been presumed that high pressure contact between the pressure roll and the single face web on the corrugating roll was necessary to provide an initial bond which would be fully gelatinized and cured by the residual heat in the two component webs produced by upstream web preheaters and heated corrugating rolls. More recent studies have indicated, however, that a high pressure nipping of the newly joined corrugated medium and single face web actually squeezes moisture from the fresh glue lines at the flute tips and, without moisture, the starch based adhesive cannot gelatinize and no bond will occur at the flute tips. Only the portions of the glue line on the flanks adjacent the flute tips gelatinize and provide the initial tack needed to hold the web together. Even so, the subsequent creation of the necessary green bond, which results from dehydration of the gelatinized glue, requires that adequate heat be supplied over a sufficient length of time. In most prior art single facers, the glued single face web is fed directly from the high pressure nip into the downstream bridge storage and initial dehydration and completion of the green bond is accomplished primarily by the residual heat in the web. However, the initial bond created at the pressure roll nip is often inadequate to assure the integrity of the glue lines as the single face web is flexed and gathered in the bridge storage area downstream from the single facer. Lack of formation of adequate green bond strength may, however, result in a single face web which will not hold together during subsequent processing through the bridge storage area, the double backer, and downstream dry end processing, the result of which may be delamination and the formation of loose back.
One recently developed apparatus and process for addressing the prior art problems is disclosed in U.S. Pat. No. 5,614,048. In that apparatus, the liner web is joined to the glued corrugated medium web on the surface of the lower corrugating roll with a pressure roll operating at a very low nip pressure. After joinder, the single face web is wrapped on the surface of the lower corrugating roll over a circumferential portion not exceeding an arc of about 80.degree.. Thereafter, the single face web is taken off the lower corrugating roll and back wrapped (or wound with the liner face of the web) around another heated roll having a smooth outer surface through a larger arc not exceeding about 200.degree.. The use of the second heated roll is necessary to provide adequate heated surface contact which is not available on the heated lower corrugating roll before the freshly glued single face web is removed therefrom. The back wrapping of the web on the heated smooth surface roll downstream from the corrugating roll necessarily takes place before adequate green bond strength has been attained in the glue lines. It is believed that the back wrapping of the single face web at this point cannot apply sufficient pressure to the glued flute tips to optimize the formation of the green bonds.